1. Field of the Invention
The present invention relates generally to an air/fuel ratio control system for a fuel injection internal combustion engine. More specifically, the invention relates to an air/fuel ratio control system which improves engine acceleration characteristics.
2. Description of the Background Art
In general, amount of fuel to be delivered to engine cylinders of an internal combustion engine is controlled based on various engine driving parameters, such as an engine load, engine revolution speed and so forth. In case of a fuel injection type internal combustion engine, fuel delivery amount is derived in a form of a fuel injection pulse having a variable pulse width for driving a fuel injection valve for a controlled period. The fuel injection pulse width Ti is derived by well known equation:
Ti=Tp.times.COEF.times.K.sub.LAMBDA +Ts
where
Tp is a basic fuel injection pulse width to be derived on the basis of an engine speed N and an engine load Q (Tp=K.times.Q/N:K is constant) PA1 COEF is a correction coefficient to be derived based on various fuel injection amount correction factors, such as acceleration enrichment demand, engine start-up enrichment demand, cold engine enrichment demand and so forth; PA1 K.sub.LAMBDA is an air/fuel ratio dependent correction coefficient for LAMBDA (.lambda.) control; and PA1 Ts is a battery voltage compensative correction value
As is well known, LAMBDA control is performed on the basis of oxygen concentration contained in an exhaust gas flowing through an exhaust passage. The correction coefficient K.sub.LAMBDA which will be hereafter referred to as "LAMBDA control correction coefficent" is derived so as to maintain the air/fuel ratio of an air/fuel mixture to be introduced into the engine cylinder at stoichiometric value. The LAMBDA control correction coefficient K.sub.LAMBDA generally comprises a proportional (P) component and an integral (I) component. At the initial stage of LAMBDA control, P component is adjusted based on the oxygen concentration indicative O.sub.2 sensor signal value from an oxygen (O.sub.2) sensor disposed in the exhaust passage. During subsequent LAMBDA control, I component is adjusted for gradually adjusting the LAMBDA control correction coefficient.
In practice, LAMBDA control is performed in CLOSED LOOP or FEEDBACK control in a predetermined stable engine driving condition, in which a predetermined FEEDBACK condition is satisfied. In the practical control, LAMBDA control is performed in LOW load and LOW engine speed range. In the engine driving range, in which the FEEDBACK condition is not satisfied, OPEN LOOP control is performed. During OPEN LOOP control, the LAMBDA control correction coefficient K.sub.LAMBDA is held at a fixed value. Such process in air/fuel ratio control has been disclosed in the Japanese Patent First (unexamined) Publication (Tokkai) Showa 58-214629.
LAMBDA control may be continued even in engine deceleration state if the engine deceleration magnitude is small enough. For example, a throttle valve angular variation in 15 miliseconds is smaller than 1.5.degree., air/fuel ratio control mode is held at LAMBDA control mode. Such engine driving condition where the engine deceleration magnitude is small enough to maintain LAMBDA control, will be hereafter referred to as "slow deceleration state". On the other hand, LAMBDA control may also be continued even in engine acceleration state when the engine speed accelerated to the engine speed range, e.g. 2400 r.p.m. to 2800 r.p.m., and approximately 60.degree. of the throttle angular position. Such engine driving state where the engine is accelerated to a speed within an engine speed range where FEEDBACK condition is still maintained, will be hereafter referred to as "small magnitude acceleration". When such LAMBDA control is applied for a single point injection type internal combustion engine, which has a single fuel injection valve to be driven once in each engine revolution cycle, air/fuel ratio tends to become lean to degrade acceleration characteristics when acceleration demand to accelerate the engine to the engine speed range where LAMBDA control is to be continued, after small magnitude of deceleration.
Namely, during small magnitude of engine deceleration, intake vacuum in an induction passage is increased to remove fuel adhering on the inner periphery of the induction passage to dry the inner periphery up and to increase fuel amount to be contained in the air/fuel mixture to be introduced into the engine cylinder. Therefore, the air/fuel ratio becomes richer than stoichiometric value. Because LAMBDA control is maintained during small magnitude of engine deceleration, rich mixture ratio causes reduction of the fuel injection amount to be injected. If acceleration demand occurs for accelerating the engine in a magnitude that maintains the engine driving condition satisfying the FEEDBACK condition, LAMBDA control is maintained to start increasing of the fuel injection amount from the value reduced during deceleration.
Furthermore, since the inner periphery of the induction passage tends to be dried up during deceleration, part of the fuel injected in response to the acceleration enrichment demand can be consumed for making the inner periphery wet. This makes the air/fuel ratio leaner to further degrade acceleration characteristics.